Python Crash Course

Ole Nielsen∗ School of Mathematical Sciences Australian National University, Canberra Ole.Nielsen@anu.edu.au April 29, 2002

Introduction
This text provides an informal introduction to selected parts of the programming language Python for use in the ANU, SMS Summer school 2002. It should take about 40 minutes to complete most of it. A more comprehensive tutorial is available at
http://www.python.org/doc/current/tut/tut.html (Chapter 3)

The site www.python.org itself contains everything worth knowing about Python. You won’t need to access any of the WEB resources given here to complete the demo — they are included for your reference only.
∗

For example to compute the interest earned in half a year on 5000 dollars with a rate of 4.org/doc/current/lib/typesseq-strings.python.principal >>> interest 118.html. Let us try print a statement from the bank. Back to the example: Let us type
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These codes are very similar to those used in the programming language C
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.59355682789101
The last command causes Python to print the result to the screen. "credits" or "license" for more information.8 >>> years = 0. Nov 20 2001.8% per year. type:
>>> principal = 5000 >>> rate = 4. For example
>>> ’Initial deposit: $%d’ %(principal) ’Initial deposit: $5000’
Notice how the value of the decimal number principal is inserted in the text instead of the placeholder %d. A key facet of Python’s string interpolation is that it can occur anywhere that a string can.1 A ﬁrst glance
The ﬁrst thing to try is to invoke the Python interpreter by typing the command python You should see the following message and the python prompt (>>>):
Python 2. This is an example of string interpolation.5 >>> interest = principal * (1+rate/100)**years .1 (#1. Python can do calculations like in most other languages. >>>
Python is now ready to take your order. A complete list of placeholders is available at http://www. 09:58:43) [C] on osf1V5 Type "copyright". Other placeholders are %f for ﬂoating point numbers and %s for strings1 . Python is often used to manipulate texts.

For now just think of it as an arbitrary text string. The special character ’\n’ is turned into a newline when the string is printed. The rounded to 2 decimals.100)]):’
This happens to be valid command in the programming language ’Maple’ which we will encounter later.>>> s = ’Initial deposit: $%d’ %(principal) >>> s = s + ’\n’ >>> s = s + ’Interest in %d months: $%.
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.59.2f. Strings can be indexed to extract any characters or substrings. Substrings can be speciﬁed with the slice notation: two indices separated by a colon. Now suppose that we wish the ﬁrst number to be taken from a variable called lower and the second from a variable upper. Write a Python statement creating such a string by replacing appropriate placeholders with the values of lower and upper respectively..’ %(years*12. The ﬁrst number (years*12) is inserted in the placeholder %d the second in %.59’
Exercise: (optional) Suppose you wish to create a string like
’primes := select(isprime.2f. interest) >>> print s Initial deposit: $5000 Interest in 6 months: $118. [seq(i. The ﬁrst character in string has index 0 and the last has index -1.’ >>> s[-4:-1] ’.i=1.
Note that text strings can be assigned to variables and concatenated using ’+’.
>>> s[0] ’I’ >>> s[0:3] ’Ini’ >>> s[3:5] ’ti’ >>> s[-1] ’.

input = popen2(’wc -w’)
If you don’t know the command wc -w.2 Pipes
Python can call upon other programs to perform tasks for it through the use of pipes. you can try to run it from the UNIX command line. then type some words. and wc will tell you how many words you typed.
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. end by typing CTRL-d. supply some input through the input pipe and retrieve the result through the output pipe. The command is used to count the number of words in the input string2 . First we need to open the pipes:
>>> from popen2 import popen2 >>> output. A pipe is essentially a channel in which one program can communicate with another by sending and receiving text strings.
Python program
Other program Inout pipe
Output pipe
A simple example is to let Python call up the standard UNIX command wc -w (word count).

%d)]):’ %(lower.The command popen2 will start the supplied program (’wc -w’). Here we go. Next thing is to is to write some text to the input pipe using the command input.50)]): print(primes):
Now we have our Maple command..they are listed at http://www. Here is the code:
>>> >>> >>> >>> input. maple_in = popen2("maple -q") >>> maple_in.write() and then close it to let the external program know that we are ﬁnished.python.i=1.read() print count 5
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>>> maple_out. This happens in the ﬁrst line.close(). issue the command and retrieve the result.1 Another — not quite as useless — example
Now suppose we wish to compute all prime numbers in some interval for our Python code.. [seq(i. This is accomplished by the command input. First we build the Maple command as a Python string using values from the variables lower and upper denoting the desired interval.html. [seq(i. popen2 is not available to the basic Python interpreter so it needs to be imported from the module of the same name. We can get the best of both worlds — Pythons power and generality combined with Maple’s specialised functions — through the use of pipes.write(s)
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.close() count = output. Python is a general purpose language and does not have primality tests.i=%d. Next thing is to open pipes to Maple. upper) >>> s = s + ’\nprint(primes):’ >>> print s primes := select(isprime. and then wait.read(). associate the two pipes with its output and input. Python comes with a very large number of such modules . To retrieve the result through the output pipe we use as you guessed output.org/doc/current/lib/modindex.write(’Now to something completely different’) input. return them to two variables on the left hand side of =. On the other hand we know that Maple which is specialised for mathematical problems can do it by using the code from the exercise above.
>>> lower = 1 >>> upper = 50 >>> s = ’primes := select(isprime.

go to the directory and write
python pipe_demo. 37.html#string-methods
Now leave the python prompt by pressing CTRL-d. for example
emacs pipe_demo.g. To execute. The call primes. You can look or edit the ﬁle with an editor.except for that .read() print primes. 7. It then proceeds to call another programming language called R which is specialised in statistics to get the mean value and the median of the primes .
3 Python scripts
Writing commands directly to Python as we have done quickly becomes tedious for complex tasks.>>> >>> >>> [2. Try printing without it. 13. 11. Python commands can therefore be collected in a ﬁle to form a Python program. 43. 23.strip() thing.e. Your directory on the Alpha Server Python demos contains a ﬁle called pipe demo.close() primes = maple_out.py &
The contents of pipe demo. 29.they are known as methods.strip() gets rid of leading and trailing spaces in strings.py is given in Section 5 for your reference. All strings (as any other type) have functions associated with them which allow us to modify them . 19. 41. dir(’asdf’)).org/doc/current/lib/string-methods. 47]
Pretty simple really .again using a set of pipes. 5. To ﬁnd out what other methods are available in the string write dir(primes) (or any string in place of primes . Documentation of each of them is available at
http://www.py
at the unix prompt.python.
maple_in. 17.strip() 3.py which is python program that calls up Maple to get all primes in a given interval using pipes as we just did.
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py
where P = 4 in this example (you can try this!).and now to something completely different If you have more time you can have a look at the program para demo.py available on the computer and in Section 6 which demonstrates how to run Python programs in parallel using message passing . The Python module pypar developed here at the ANU implements message passing parallelism for Python.rank().
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. The program is started on all P processors using the command
prun -n 4 para_demo.. Each program must ﬁnd out which number it is and how many processors are being used. The program then prints a statement about who it is to the screen.a paradigm where processors communicate by explicitly sending and receiving data to and from each other. This is done in the beginning of the program in the statements numproc = pypar. In this demo a message is passed in a ring from processor 0 to processor 1 then on to 2 and so forth until the last processor sends a message back to processor 0. The processors are numbered from 0 to P − 1 where P is the number of processors in use.size() and myid = pypar. Not in itself a very useful parallel program but often used to measure communication speed on parallel machines..4 Parallel Python
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Here you also see an example of an if-statement in Python. It ﬁrst identiﬁes the preceding processor (source) and the following processor. it evaluates to myid+1 except in case of the last processor (myid = numproc-1) where it will evaluate to 0 as desired.The program running on processor 0 initiates this communication as follows:
if myid == 0: msg = "Anybody out there?" pypar. The expression (myid+1)%numproc computes myid+1 modulo numproc i. And that is really all there is to it. Then it issues the receive command and waits. destination) pypar.e. the indented code following if is only executed in the program running on processor 0. When the message arrives it adds its own id to it (as an example) and passes it on to destination using the send command. After having sent its message processor 0 issues the command
msg = pypar. All other processors must receive a message from the one preceding them and pass it on to their successor (or processor 0 in case of the last one). msg. 1)
It makes a message and sends it to processor 1 using the command send for the module pypar.receive(source) msg = msg + ’ P’ + str(myid) + ’_OK’ print ’Processor %d sending msg "%s" to %d’ %(myid.
Hope you enjoyed the demo ! 8
. The hard part in parallel programming. But that is a different story. destination)
The else: statement indicates that the following indented code is only executed if myid = 0.send(msg. The code is
else: source = myid-1 destination = (myid+1)%numproc msg = pypar. is to arrange communications and computations to make the parallel program more efﬁcient than the sequential counterpart. however.send(msg.receive(numproc-1)
which puts it to sleep until the last processor sends it something.

# # # The code invokes Maple to generate a vector of all primes # in a given interval [lower. ANU.5 pipe demo. upper] # # The result is then transformed into a set of R commands # form computing the mean and the median of those primes.
#!/bin/env python ######################################################### # # ANU Summer School in Computational Mathematics 2002 # # Example: Using Python to invoke Maple and R using pipes. # Understanding pipes is crucial for appreciating the # Master-Slave program presented on Thursday 17 January 2002. # # Author: Ole Nielsen.py
Here is the Python (+Maple and R) source code for the pipe demo. Jan. SMS. # R is invoked and the results are written to the screen
from popen2 import popen2
# The pipe function
# Interval in which prime numbers are generated. # lower = 1 upper = 50
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. # ######################################################### # # The purpose of this code is to demonstrate how Python can be # used to combine functionality from foreign packages into # one single program by using bi-directional Unix pipes. 2002. Lines starting with # are comments not executed by Python.